Mass spectroscopy for samples, such as organic compounds, uses mass spectrometers. A mass spectrometer typically includes an ion source, a mass spectrometry unit, and a data processing unit, for example. The mass spectrometry unit uses an electric field and/or a magnetic field to separate or extract individual ions in accordance with m/z of the individual ions: where m is mass of the ion and z is electric charge. The data processing unit generates a mass spectrum based on the detection results by the mass spectrometry unit. Known mass spectroscopy methods include time-of-flight mass spectroscopy, quadrupole mass spectroscopy, double focusing mass spectroscopy, ion trap mass spectroscopy, and ion cyclotron mass spectroscopy, for example.
The ion source ionizes a sample. Known ionization methods include Electron Ionization (EI), Chemical Ionization (CI), Atmospheric Pressure Chemical Ionization (APCI), and Electro Spray Ionization (ESI), for example.
Direct Analysis in Real Time (DART) has been widely used. DART generates plasma from helium gas using corona discharge, for example, and extracts neutral gas molecules in an excited state out of the plasma and irradiates a sample with the neutral gas molecules to produce a positive ion or a negative ion of the sample. (See JP 2014-215078 A, for example). DART, which is one type of APCI and can be used to analyze various ambient substances without pretreatments, is regarded as one ambient analysis technology.
When a gas chromatography apparatus or a liquid chromatography apparatus, for example, is connected to a mass spectrometer, an ion source in accordance with the ionization method including EI, ESI, or APCI described above is used as an interface.
As there are no ionization methods that are adaptable to all substances, a user needs to select an ionization method in accordance with a sample. Specifically, to perform mass spectroscopy with high precision, an appropriate ionization method which is suitable for a sample substance must be selected. It is known, for example, that ESI is not typically suitable for substances with low polarity, and that EI and APCI are not typically suitable for substances with high polarity and thermally unstable substances. While DART is known to be suitable for a relatively wide range of polarity, it is difficult to definitely specify the upper or lower limit of polarity.